/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineFriendsInterface.h
Checks that a unique player id is part of the specified user's friends list
true if friends list was found and the friend was valid
bool IsFriend ( int32 LocalUserNum, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & FriendId, const [FString](API\Runtime\Core\Containers\FString) & ListName )